Ljubljana, 4 September - Slovenian scientists have recently discovered high-temperature spin liquid in layered crystal TaS2 (tantalum(IV) sulfide). What is special about the discovery is that spin liquid can survive relatively high temperatures that can be produced fairly easily in a laboratory.

This brings important implications for application in quantum technologies, explains scientist Martin Klanjšek. An article about the discovery has been published in the high-profile scientific journal Nature Physics.

Quantum magnets such as layered crystal TaS2 are constituted of angular momentums or spins. In a majority of cases, spins are completely disordered at sufficiently high temperatures, and their state can roughly be compared with the gaseous state of matter.

If such a magnetic system is sufficiently cooled, spins get in order, similarly to atoms in a crystal in the solid state of matter, explained one of the authors of the article, Klanjšek, a researcher at the Jožef Stefan Institute (IJS) and teacher at the Ljubljana Faculty of Mathematics and Physics (FMF).

Members of three departments at the IJS and FMF have established that spins in layered crystal TaS2 do not get in order even at the temperature of -273 degrees Celsius (almost at absolute zero) and that they actually form a new state of quantum spin liquid, which is an analog of the liquid state of matter. What is more, they have proven that spin liquid also persists at very high temperatures.

Quantum mechanics is usually observed at extremely low temperatures, while spin liquid discovered by the Slovenian scientists can also survive at temperatures around -100 degrees Celsius, which is an exceptionally high temperature for quantum mechanics.

Such quantum state can therefore be achieved at temperatures which can be relatively simply secured in a laboratory by applying widely-available liquid nitrogen, which brings important implications for the development of quantum technologies, explained Klanjšek.

The first to predict spin liquid was Nobel laureate P. W. Anderson more than 40 years ago, when he discussed the unusual magnetic properties of layered crystal TaS2.

While magnetic states which would be an analog of the liquid state of matter had been announced in theory, they have so far rarely been observed in practice, and only at extremely low temperatures, in the proximity of absolute zero (-273.15 degrees Celsius).

Quasiparticles opening new possibilities

As Klanjšek explains, spin liquids are interesting in their own right, because they represent a new state of magnetic matter, and are even more important because of very unusual elementary excitations or quasiparticles, which are produced in this state. According to Klanjšek, quasiparticles are a concept that allows us to understand the behaviour of any matter at temperatures above absolute zero (-273.15 degrees Celsius).

The concept of quasiparticle was introduced by Nobel laureate Lev Landau when researching superfluidity of helium. While perhaps the best-known example of a quasiparticle is a photon or a "particle" or a quantum of light, scientists discovered a number of very interesting quasiparticles in the last decade.

"These quasiparticles are interesting primarily because they had been announced in theory, but never discovered in practice as independent particles. What is more, application in practice has been proposed for almost all of them. They are different enough from the previously known quasiparticles for their properties to potentially open new possibilities," explained Klanjšek.

Quasiparticles are important for development of quantum computing

An interesting type of quasiparticles which will most probably be important for the development of quantum technologies are anyons. The name is derived from the English word "any", which refers to the difference from other known particles and quasiparticles which can be classified either among bosons (such as photons as "particles" of light) or fermions (such as electrons).

"When anyons are exchanged or when 'one particle is taken around another one', the state of a pair of particles changes fundamentally, which otherwise does not apply to bosons or fermions. We could say that a knot is tied between them, which is a very stable formation in the topological sense," Klanjšek stressed. If states such as knots are found, this is "something spectacular".

Due to their stability, anyons are important for the development of quantum computing, because a knot can be used as a bit of information. Such a bit will be persistent and will not eventually disintegrate. The main problem of quantum computing is exactly the non-persistence of quantum states at absolute temperatures and poor isolation of the quantum system from the environment.

"The word is out that Microsoft is developing a quantum computer based on anyons. Other major technology companies, such as Google and IBM, use different platforms for the development of quantum computers which are not completely quantum in their nature," said Klanjšek.

New research by IJS scientists promises important results

For the time being, there are only two potential realisations of anyons, but neither of them is believed by scientists to actually realise anyons.

IJS scientists are most probably about to make a breakthrough in this field, as their latest experiments imply that magnetic anyons in RuCl3 (ruthenium(III) chloride) do exist, according to Klanjšek.

This is a material which has been used for decades as the starting compound for a great deal of chemistry based on ruthenium. Very little had been known until a few years ago about the element's physical properties, especially magnetic properties.

A few years ago, scientists became aware that this material has all the required properties for the fantastic proposal by physicist Alexei Kitaev from 2006 on the occurrence of magnetic anyons to be realised. This material is currently being examined by many experimental groups around the world, including by IJS scientists. An article about their achievement is currently being reviewed.

It is important to combine research and teaching

In addition to research work at the IJS, Klanjšek teaches at the FMF. He finds it important that the two fields are combined. Research is a long-term process, which is why satisfaction comes only when you make a discovery and publish an important thing. Teaching has a reverse logic: if a lecture is successful, you can see it on the faces of students and you leave the classroom happy after two hours of work.

Klanjšek is critical about how science and scientists are presented in and perceived by the public. In his view, the public frequently perceives science as spending public funds, while not being aware that it takes a great effort to acquire funds for financing research. Applying for calls for applications for the funding of research projects takes ever more time, with certain statistics showing that scientists spend a third of their time on administrative work.

The scientist stressed that research is a long-term process in which you sometimes spend long months in a laboratory, rack your brains and make experiments but nothing ever happens. But this time is not wasted, because it is time when small steps are being taken, which cannot be evaluated but which can eventually lead to a major discovery.

"The charm of research work is exactly in not knowing what you will discover. You discover new things that you couldn't have predicted or imagined, which is why the result of this work has some value too. Science in general is one of the rare activities which allows you to arrive at something new."

About Martin Klanjšek

Martin Klanjšek earned a PhD degree in physics in 2004, receiving the Futurum award for a breakthrough doctoral thesis and a Jožef Stefan Golden Emblem Prize from the IJS. In 2007, he conducted post-doctoral research at the Grenoble High Magnetic Field Laboratory in France, while between 2008 and 2011 he was a guest researcher at the Joseph Fourier University in Grenoble. He has been cooperating with the IJS's condensed matter physics department since 1999, becoming a research fellow there in 2008. Since 2000, he has also been a research assistant at the Ljubljana Faculty of Mathematics and Physics.